In a refrigerant cycle device for a vehicle described in JP-A-2000-335232, a variable displacement mechanism is provided for changing a discharge capacity (displacement) of a compressor based on a control signal from an exterior. In this refrigerant cycle device, when a controller determines an accelerating state of the vehicle engine, the compressor is stopped for a predetermined time, and thereafter, the variable displacement mechanism is set at a partial capacity state so that the compressor operates with a partial discharge capacity. After the compressor operates with the partial discharge capacity, the compressor operates with 100% discharge capacity.
Accordingly, immediately after an accelerating start of the vehicle engine, the compressor driving power by the vehicle engine becomes zero, and the accelerating property of the vehicle can be improved. Then, because the compressor operates with the partial discharge capacity, a refrigerant circulating amount in the refrigerant cycle device can be maintained at a predetermined flow amount, thereby reducing a temperature increase in air blown into a vehicle compartment. However, according to studies of the present inventors, the following problem may be caused in this control of the compressor.
FIG. 8 shows a swash-plate type compressor 2 having a variable displacement mechanism (control valve) 15. The variable displacement mechanism 15 adjusts the pressure in a crank chamber 22 so as to change a tilted angle of a swash plate 21 and control the discharge capacity of the compressor 2. Furthermore, as shown in FIG. 9, the variable displacement mechanism (control valve) 15 includes a valve body 15b and an electromagnetic coil 15a for pushing the valve body 15b, and is connected to the crank chamber 22, a discharge chamber 24 and a suction chamber 23 through passages 25 in the compressor 2. In the variable displacement mechanism 15, by using a balance between the pushing force Pf of the electromagnetic coil 15a and the suction pressure Ps from the suction chamber 23, the open degree of the valve body 15b positioned in the passage 25 between the crank chamber 22 and the discharge chamber 24 is adjusted, thereby adjusting the pressure Pc of the crank chamber 22.
For example, when the suction pressure Ps is larger than a predetermined pressure value, the valve body 15b moves in a valve-closing direction. In this case, the passage 25 from the discharge chamber 24 to the crank chamber 22 becomes in a shutting state, thereby the discharge capacity becomes larger and the suction pressure Ps is lowered. In contrast, when the suction pressure Ps is smaller than the predetermined pressure value, the valve body 15b moves in a valve-opening direction. In this case, high-pressure refrigerant flows from the discharge chamber 24 to the crank chamber 22, thereby the discharge capacity becomes smaller and the suction pressure Ps is increased. As a result, the suction pressure Ps can approach a predetermined value.
Because the compressor 2 has the above characteristics, a necessary discharge capacity of the compressor 2, to be controlled in accordance with a thermal load of an evaporator, is different, and the driving power of the compressor 2 is also different, even when the control current value is made the same. For example, when the thermal load of the evaporator is small, the suction pressure Ps becomes lower, In this case, the necessary discharge capacity of the compressor 2 is small, and the driving power of the compressor 2 becomes small. In contrast, when the thermal load of the evaporator is large, the suction pressure Ps becomes higher, In this case, the necessary discharge capacity of the compressor 2 for obtaining the suction pressure Ps is large, and the driving power of the compressor 2 becomes large.
Thus, at a time where the accelerating control of the vehicle engine is performed, when the thermal load is changed, the compressor driving power is not determined only in accordance with a decrease of the control current. For example, if the control current is lowered when the thermal load is small, the discharge capacity may be excessively lowered. In this case, a time for increasing the discharge capacity after this control becomes longer, and unpleasant feeling may be given to an occupant in a vehicle compartment. In contrast, if the control current is lowered when the thermal load is large, the discharge capacity of the compressor cannot be sufficiently decreased, and consumed power of the vehicle engine cannot be effectively reduced.